In the chemical surface treatment of metals, for example as a preparation for the application of lacquers, adhesives and other synthetic or resinous materials, increasing importance is being given to so-called 3-stage methods. In the first stage of such methods, the surface of the metal is cleaned in order to eliminate oil, dirt and corrosion products; in the second stage, the metal surface is rinsed with water in order to remove any chemical residues left by the first stage; finally, in the third stage, the metal surface is wetted with an aqueous chemical reaction solution, and the film of liquid is then dried. This leaves, on the metal, a thin, non-metallic coating which can effect a decisive improvement in surface quality, if the composition of the solution and the reaction conditions are appropriate. Coatings of lacquers, adhesives and other synthetic or resinous materials, for example, may therefore adhere better and may provide considerably better corrosion protection, if they are applied to metal thus pretreated.
German Public Inspection Text No. 17 69 582 described a method in which an aqueous solution containing hexavalent chromium, trivalent chromium, alkali cations and silicon dioxide in specific ratios is dried upon the metal. The coatings formed are highly suitable as electrical insulation, protection against corrosion and as an adhesive base for lacquers and the like. One major disadvantage of this method, however, is the presence of hexavalent chromium which, because of its toxic and carcinogenic properties, makes special precautions necessary during application of the layer-forming solution and in the handling of the coated metal.
U.S. Pat. No. 2,030,601 discloses another method in which highly concentrated aqueous solutions, containing from 10 to 20% of phosphoric acid, from 10 to 15% of sodium dichromate and, possibly, silicic acid, are brushed onto iron surfaces and are then dried. This treatment provides protection against the formation of rust, but here again the known precautions for handling hexavalent chromium must be taken. The use of chromium salts, in which the chromium is present in the form of the base ion, is not advised because the corrosion protection provided thereby is inadequate and the appearance of the coating is unsatisfactory.